Low carbon ni-cr alloy steel having an improved resistance to stress corrosion cracking

ABSTRACT

Herein disclosed is a low-carbon Ni-Cr alloy steel being substantially in the single phase of austenite and improved in resistance to stress corrosion cracking. The alloy steel consists essentially of, by weight, less than 0.03% of carbon, 1.5 to 4.0% of silicon, 0.1 to 3.0% of manganese, 25 to 45% of nickel, 24 to 35% of chromium, 0.9 to 4.0% of vanadium and, if desired, copper and/or molybdenum in a total amount of 0.3 to 4.0%, and the balance of iron. This alloy steel has outstanding utility as a structural material of heat exchanger, mainly for use in the nuclear reactor.

United States Patent 1191 Kowaka et al.

[ Dec. 16, 1975 LOW CARBON NI-CR ALLOY STEEL HAVING AN IMPROVED RESISTANCE TO STRESS CORROSION CRACKING [75] Inventors: Masamichi Kowaka, Toyonaka;

Hisao Fujikawa, Nishinomiya, both 21 Appl. No.: 440,470

Related US. Application Data [63] Continuation-impart of Ser. No. 161,289, July 9,

1971, abandoned.

[30] Foreign Application Priority Data July 14, 1970 Japan 45-62463 [52] US. Cl 75/122; 75/125; 75/128 C; 75/128 V; 75/128 W; 75/134 F [51] Int. Cl? ..C22C 38/46; C22C 38/54;

[58] Field of Search. 75/125, 128 A, 128 C, 128 V, 75/128 W, 122, 134 H [56] References Cited UNITED STATES PATENTS 2,193,222 3/1940 Browne 75/128 V 2,750,283 6/1956 Loveless 75/128 C 3,152,934 10/1964 Lula 3,674,468 7/1972 Ota 75/125 OTHER PUBLICATIONS ASM Metals Handbook, Vol. I, 1961, ed., p. 409.

Primary E.\'aminerL. Dewayne Rutledge Assistant ExaminerArthur J. Steiner Attorney, Agent, or FirmBucknam and Archer [5 7] ABSTRACT Herein disclosed is a low-carbon Ni-Cr alloy steel being substantially in the single phase of austenite and improved in resistance to stress corrosion cracking. The alloy steel consists essentially of, by weight, less than 0.03% of carbon, 1.5 to 4.0% of silicon, 0.1 to 3.0% of manganese, 25 to 45% of nickel, 24 to 35% of chromium, 0.9 to 4.0% of vanadium and, if desired, copper and/or molybdenum in a total amount of 0.3 to 4.0%, and the balance of iron. This alloy steel has outstanding utility as a structural material of heat exchanger, mainly for use in the nuclear reactor.

3 Claims, No Drawings LOW CARBON NI-CR ALLOY STEEL HAVING AN IMPROVED RESISTANCE TO STRESS CORROSION CRACKING This is a continuation-in-part of US. Pat. Application Ser. No. 161,289 filed on July 9, 1971 now abandoned.

This invention relates to a low-carbon Ni-Cr alloy steel improved in resistance to stress corrosion cracking, and more particularly to a low-carbon Ni-Cr alloy steel improved in resistance to the stress corrosion cracking that is liable to occur in pure water, chlorine ion containing water or vapour thereof at high temperature and high pressure.

In recent years, demand for stainless steels or alloy steels excellent in resistance to stress corrosion cracking, particularly in the application to heat exchangers, has increased with the development of the nuclear and chemical industries. For instance, in power reactors, the heat exchanger is employed to transfer heat from the reactor coolant to the working fluid used in power generation. 1n the case of pressurized water reactors, the heat exchangers transfer heat from the reactor coolant to a secondary water steam. And in the case of boiling water reactors, steam is generated in the reactor vessel proper. However, in some designs an external heat exchanger is employed to generate additional steam. In these cases the heat exchangers are subjected to a severe corrosive environment of pure water, chlorine ion containing water or vapor thereof at high temperature and high pressure, for instance, at temperatures up to about 350C and pressures up to about 200 kglcm In view of the radioactivity associated with reactor Coolants, leakage specifications in such heat exchanger are very stringent.

It is well known that heat exchangers made of Ni-Cr stainless steels or alloy steels, for example U-shaped tubes, are liable to suffer stress corrosion cracking, especially in chlorine ion containing environment of high temperature and high pressure. There have been proposed many measures for overcoming this defect, such as removal of residual stress from welded or worked pieces, improvement of corrosive enviroment and of design of pieces, surface processing such as shot peening, and employment of Ni-alloy containing a large percentage of nickel like Inconel (trade name).

These measures, however, are expensive to conduct and as such, are unsatisfactory from a commerical point of view.

Accordingly, the object of the invention is to provide an Ni-Cr alloy steel improved in resistance to stress corrosion cracking.

It is a further object of the invention to provide an Ni-cr alloy steel improved in resistance to the stress corrosion cracking that is liable to occur in pure water, chlorine ion containing water or vapor thereof at high temperature and high pressure.

It is a still further object of the invention to provide a long lasting heat exchanger for a nuclear reactor.

Other objects and advantages of the invention will become apparent from the following description.

After a considerable amount of research, we discovered that mode of stress corrosion cracking in Ni-Cr stainless steel or alloy steel varies highly depending on conditions of the corrosive environment which is surrounding the steel. There "has usually been employed boiling magnesium chloride solution as a test medium for examining stress corrosion cracking. In this test medium, the stress corrosion cracking of Ni-Cr stainless steel or alloy steel is observed mainly as intergranular cracking. On the other hand, the corrosive environment in which the heat exchangers for a nuclear reactor is positioned, that is, pure water, chlorine ion containing water or vapor thereof at high temperatures and high pressures causes in Ni-Cr stainless steel or alloy steel a stress corrosion cracking appearing not only as intergranular cracking but also as transgranular cracking. As such, the stress corrosion cracking in the latter case cannot be estimated from the test result employing boiling magnesium chloride solution as a test medium.

We have found that the susceptibility of Ni-Cr stainless steel or alloy steel to the stress corrosion cracking in the latter case is markedly affected by the content of carbon dissolved in solid solution of the steel. Particularly, more than 0.03 weight of carbon adversely affects the resistance to stress corrosion cracking of Ni-Cr stainless steel or alloy steel. We suppose that this degradation in resistance to stress corrosion cracking is caused by formation of chromium carbide Cr C in the vicinity of thegrain boundary on heating the steel to a temperature of about 500 to 800C and the resultant chromium-depleted area at that portion. Such effect of carbon is not observable in Ni-Cr stainless steel or alloy steel located in the corrosive environment of boiling magnesium chloride.

We also discovered that addition of vanadium to Ni-Cr stainless steel or alloy steel remarkably increases the resistance to stress corrosion cracking in the corrosion environment of pure water, chlorine ion containing water or vapor thereof at high temperature and high pressure. It was also found that addition of silicon is effective for preventing transgranular cracking although not effective for preventingintergranular cracking. For preventing the stress corrosion cracking, it is also preferable to make the steel structure austenite.

Thus, according to the present invention, there is provided a low carbon nickel-chromium alloy steel being substantially in a single phase of austenite and improved in resistance to stress corrosion cracking, consisting essentially of less than 0.03% of carbon, 1.5 to 4.0% of silicon, 0.1 to 3.0% of manganese, 25 to 45% of nickel, 24 to 35% of chromium, 0.9 to 4.0% of vanadium and the balance essentially of iron.

The reason for the restriction on the chemical composition is as follows:

The content of carbon is restricted to less than 0.03% because it increases the susceptibility to stress corrosion cracking as described in the above.

Regarding the silicon content, it is usually incorporated in steels in an amount of up to about 1% for deoxidation of steels. But, the steel of the present invention requires a silicon content of not less than 1.5%, because with a silicon content of less than 1.5%, a sufficient resistance to transgranular cracking is not obtainable and the resulting steel cannot withstand the 2,000 hour corrosion test as described hereinafter. On the other hand, a silicon content of more than 4.0% adversely affects the weldability and workability of the resulting steel.

Regarding manganese, a content of less than 0.1% offers some difficulties in deoxidation and hot working of the steel and, on the other hand, a content exceeding 3.0% presents some problems in steel making and working.

3 In order to maintain the steel structure substantially in the single phase of austenite. the Ni-Cr stainless steel of the present invention preferably contains nickel content of not less than 25% and chromium content of not tance. But. a molybdenum and/or copper content in a total amount exceeding 4.0 weight adversely affects the resistance to stress corrosion cracking.

The present invention willbe further illustrated by less than 24%. Regarding nickel, it serves to make the way of an example which is to be only for illustration austenite structure stable and affords resistance to and does by no means limit the scope of the invention. stress corrosion cracking. In fact, the higher the nickel PLE content, the more effectively the resistance to stress corrosion is enhanced. However, since nickel is an ex- Test specimens each having a shape of 2 mm X pensive element. the upper limit of the nickel content is 10 mm X 75 mm and chemical compostion indicated in set at weight 7: from the economical view point. Re- Table 1 were prepared. Speciments A-l to A-7 were garding chromium, it serves also to afford resistance to prepared according to the most preferred embodiment stress corrosion cracking. But, chromium content ex of the present invention. specimens 8-] to 8-8 had ceeding 35% presents difficulties in working of the chemical compositions similar to those of specimens steel. A-l to A-7. in fact, the steels of 8" group had many Regarding vanadium, it serves to afford resistance to alloy elements which served to afford the resistance to stress corrosion cracking. Particularly in order to obstress corrosion cracking. The effect derived from the tain an enhanced resistance to stress corrosion crackdifferences in chemical composition between the steels ing, the Ni-Cr alloy steel of the present invention reofA group and the steels ofB group will be shown quires a vanadium content ofat least 0.9 weight But. 30 hereinafter in the instant Example. Specimens C-l to a vanadium content of more than 4.0 weight ad- C-l3 were control steels for showing the distinguished versely affects the weldability of the resulting steel. advantages of the present invention. Specimens D-l to In addition to the above reason, the chemical compo- D-lO were the commercially available steels which sition of the steel of the present invention is set up so were known for their resistance to the normal corrosive that the resulting steel has a sufficient resistance to 25 attack.

Table 1 Chemical Composition (/z) Steel No. C Si Mn Cu Ni Cr Mo V A A-l 0.011 2.14 1.53 0.014 001 34.58 25.13 2.17 2.08 A-2 0.008 1.83 L46 0009 0.01 26.92 25.13 0.01 1.32 A-3 0.005 2.16 L62 0.008 0.01 26.86 24.93 0.01 1.89 A-4 0.012 1.92 1.51 0.006 0.01 35.46 25.23 0.01 1.48 A-S 0.004 2.23 1.38 0.012 0.01 35.10 24.92 2.26 1.22 A-o 0.0ll 1.96 1.42 0011 l .54 35.08 25.02 0.01 l.5l A-7 0.012 2.08 1.53 0.011 0.82 33.49 25.32 2.01 1.53

01 0.021 0.07 1.44 0.014 0.01 34.63 24.61 0.01 02 0.013 1.51 1.40 0.011 0.01 24.42 20.47 0.01 C6n1r61 03 0.017 2.72 1.34 0.013 0.01 16.09 18.03 0.01

steels G4 0.07 0.58 1.28 0.015 0.01 34.72 24.49 0.01 0.98

Inconel D-l 0.03 0.28 0.19 0.005 0.01 75.24 15.63 0.01 0.01 Incoloy 13-2 0.05 0.62 1.22 0.023 0.06 33.05 21.12 0.01 0.01 Marketed D-3 0.06 0.65 1.78 0.025 0.07 9.25 18.65 0.06 austenitic D-4 0.06 0.54 1.66 0.023 0.03 13.30 16.50 2.14

stainless D-5 0.06 0.60 1.75 0.022 0.07 11.10 17.50 0.06 Ti 0.44 steel 13-6 0.05 0.66 1.69 0.024 0.07 11.70 17.60 0.07 NbO 71 molybdenum and copper does not substantially affect,

the resistance to stress corrosion cracking. A molybdenum and/or copper content in a total amount of less than 0.3 weight does not afford any significant effect on the improvement of the ordinary corrosion resis- Each specimen was subjected to the solution treatment (specimens A-l to A-7, 13-1 to B-8 and C-1 to C-7 in Table l were water cooled after heating to 1,150C, speciment D-l water cooled after heating to 920C; specimen D-2 water cooled after heating to l,l50C; specimens D-3 to D-6 water cooled after heating to 1,100C) and another set of specimens shown in Table l were subjected to the sensitization treatment by aircooling after heating to 677C for 5 hours. Pair of the thus treated specimens were laid over one another and bent with a mandrel of 7.5 mm in radius into a double U shape. These specimens were tested to evaluate the degree of the resistance to stress corrosion cracking in water and steam at 300C which contained chlorine ion (550 ppm; added as NaCl) and had been saturated with dissolved oxygen (at room temperature). The tests were continued under substantially the same condition maintained by changing the chlorine ion containing water every 100 hours until the occurence of cracking was observed and up to 1,000 hours if it was not observed. The result of the above test is illustrated in Table 2.

6 fall within the scope of the most preferred embodiment of the present invention did not suffer any cracking up to 2,000 hours under the severe conditions of the above test, while specimens 8-1 to B-S whose carbon content was very low but whose chemical compositions were outside the scope of the most preferred embodiment of the present invention did not exhibit such an excellent Table 2 Evaluation Time to cracking (hr) Phase Liquid phase Vapor phase Heat Solution Sensitization Solution Sensitization No. treatment treatment treatment treatment treatment A-l NC NC NC NC NC NC NC NC A-2 NC NC NC NC NC NC NC NC A-3 NC NC NC NC NC NC NC NC A-4 NC NC NC NC NC NC NC NC A-5 NC NC NC NC NC NC NC NC A-6 NC NC NC NC NC NC NC NC A-7 NC NC NC NC NC NC NC NC 8-] NC NC NC NC NC NC NC NC B-2 NC NC NC NC NC NC NC NC B-3 NC NC NC NC NC NC NC NC B-4 NC NC NC NC NC NC NC NC B-S NC NC NC NC NC NC NC NC 8-6 NC NC NC NC NC NC NC NC B-7 NC NC NC NC NC NC NC NC B-S NC NC NC NC NC NC NC NC Cl 600 800 400 500 600 600 400 400 C-2 NC 800 500 600 800 900 600 600 Control 03 NC NC 600 600 900 i000 600 600 steels C-4 600 700 300 300 500 700 300 200 C-5 600 800 200 200 500 600 200 100 C-6 700 700 200 100 600 400 200 300 C-7 600 400 300 300 500 500 300 400 lnconel D-l NC NC 300 500 NC NC 500 500 lncoloy D-2 500 400 200 200 500 300 200 200 Marketed D-3 100 100 100 I00 100 200 100 100 austenitic D-4 200 200 100 I00 100 100 [00 100 stainless D-S 100 100 100 100 100 I00 I00 100 steels D-6 100 200 100 100 100 200 100 100 Note: NC indicated in the Table represents the case where cracking does not appear in a l000-hour test.

Regarding specimens A-l to A-7, and B-1 to B-8, the above test was further continued up to 2000 hours and the result in this case is illustrated in Table 3.

Table 3 2000 Hours Test Time to cracking (hrs) Note: NC indicated in the Table represents the case where cracking did not ap' pear after the lapse of 2000 hours.

As seen from Tables 2 and 3. specimens of the groups A" and B which were not only solution treated but also sensitization treated, did not exhibit cracking up to 1,000 hours in either liquid or gas phase. Particularly. specimens A-] to A-7 whose chemical compositions resistance to stress corrosion cracking as specimens A-l to A-7. When the carbon content is large or the va nadium content is little such as in the case of control steels G1 to C-7, stress corrosion cracking occurred almost in both liquid and gas phases. lnconel, which had been known as the best material for the resistance to stress corrosion cracking, suffered cracking in both liquid and vapor phase after the elapse of 300 to 500 hours due to sensitization treatment. lncoloy and other marketed stainless steels, not only sensitization treated but also solution treated, displayed cracking within several hundred hours in the above test.

We claim:

1. A low carbon Ni-Cr alloy steel being substantially in the single phase of austenite and improved in resistance to stress corrosion cracking, consisting essentially of, by weight, less than 0.03% of carbon, 1.5 to 4.0% silicon, 0.1 to 3.0% of manganese, 25 to 45% of nickel, 24 to 35% of chromium, 0.9 to 4.0% of vanadium and the balance essentially of iron.

2. A low carbon Ni-Cr alloy steel being substantially in the single phase of austenite and improved in resis tance to stress corrosion cracking, consisting essentially of, by weight, less than 0.03% of carbon. 1.5 to 4.0% of silicon, 0.l to 3.0% of manganese, 25 to 45% of nickel, 24 to 35% of chromium, 0.9 to 4.0% of vanadium and at least one selected from the group consisting of copper and molybdenum in a total amount of 0.3 to 4.0%, and the balance essentially of iron.

3. An alloy according to claim 1 comprising at least one of copper and molybdenum, the total amount being up to 4%. 

1. A LOW CARBON NI-CR ALLOY STEEL BEING SUBSTANTIALLY IN THE SINGLE PHASE OF AUSTENITE AND IMPROVED IN RESISTANCE TO STRESS CORROSION CRACKING CONSISTING ESSENTIALLY OF, BY WEIGHT, LESS THAN 0.03% OF CARBON, 1.5 TO 4.0% SILICON, 0.1 TO 3.0% OF MANGANESE, 25 TO 45% OF NICKEL, 24 TO 35% OF CHROMIUM, 0.9 TO 4.0% OF VANADIUM AND THE BALANCE ESSENTIALLY OF IRON.
 2. A low carbon Ni-Cr alloy steel being substantially in the single phase of austenite and improved in resistance to stress corrosion cracking, consisting essentially of, by weight, less than 0.03% of carbon, 1.5 to 4.0% of silicon, 0.1 to 3.0% of manganese, 25 to 45% of nickel, 24 to 35% of chromium, 0.9 to 4.0% of vanadium and at least one selected from the group consisting of copper and molybdenum in a total amount of 0.3 to 4.0%, and the balance essentially of iron.
 3. An alloy according to claim 1 comprising at least one of copper and molybdenum, the total amount being up to 4%. 